1. Field of the Invention
The present invention relates to a printing head for dot matrix printers and, more particularly, to the structure of an improved printing head which is capable of being assembled and disassembled with facility.
2. Description of the Prior Art
Dot matrix printers which protrude a plurality of printing wires toward recording paper in accordance with printing information so as to form desired characters symbols, etc., by a plurality of dots are known and used as output devices for various information apparatus.
In a printing head of such a dot matrix printer, a plurality of printing wires are ordinarily so arranged that the tips thereof are situated on the circumference of a circle or a circumference of concentric circles. During printing, every time the position of a character is reached, a desired number of printing wires are operated so as to constitute dots, and a group of these dots form the character.
Such a type of conventional printing head (U.S. Pat. No. 4,225,250) for dot matrix printers is shown in FIGS. 4 to 6.
This printing head has a cylindrical sandwich structure, and is composed of a base plate 10, a ring magnet 12, a hammer disc 14, a face plate 16, column members 18 and coils 20 wound around the column members 18.
The ring magnet 12 is a permanent magnet which is attached to the base plate 10, and the magnetic field thereof is divided into magnetized portions 22 and non-magnetized portions 24 at axially regular intervals. Holes 40 for attaching the ring magnet 12 are made in each of the non-magnetized portions 24. The base plate 10 is provided with a plurality of tapped holes 26 for receiving respective column members 18, as shown in FIGS. 5 and 6.
The column member 18 is made of, for example, permeable material, and the length thereof is substantially equal to the thickness of the ring magnet. The coil 20 is wound around each of the column member 18.
The hammer disc 14 is attached to the ring magnet 12 through spacers 28 and 30 of a permeable material. The hammer disc 14 is made of an elastic permeable material and is provided with a plurality of hammers 32 at regular intervals in the radially inward direction of the disc 14. The number of the hammers 32 is equal to the number of the magnetized portions 22 of the ring magnet 12. The free ends of the hammers 32 carry printing wires 34-1, 34-2, . . . .
When assembling such a printing head, the coil 20 is first wound around the column member 18, which is screwed into the base plate 10. The base plate 10, the ring magnet 12, the spacer 28, the hammer disc 14, the spacer 30, and the face plate 16 are laid with one on top of another in that order, and bolts 36 are inserted in this state so as to be bolted in the threaded holes 38 provided on the peripheral portion of the base plate 10.
The above-described structure enables a small-sized printing head to be provided with inexpensive manufacturing cost.
Such a conventional printing head, however, has some problems. For example, when the printing head is to be disassembled, removal of the hammer disc is difficult, and the printing quality is deteriorated by magnetic interference.
Since the printing head adopts the integral type of ring magnet 12, when parts must be removed because of defect of a part, etc. after the completion of assembly, the strong magnetic force makes it difficult to separate the part from the ring magnet 12, and since the hammer disc 14 and the like are fixed to the ring magnet 12, the ring magnet 12 cannot be removed without disassembling all the parts.
Removal of parts when the magnetic force exists involves a risk of damaging a leaf spring and the like which requires accuracy.
In addition, since the magnetic path of the coil 20 does not constitute a closed magnetic circuit, excitation of the coil 20 produces magnetic interference. Therefore, if a single printing wire is driven, the magnetic flux of the coil 20 can pass through magnetic paths constituted by other hammers, cores and the like, but when a plurality of printing wires are driven, the number of magnetic paths through which the magnetic flux can pass becomes smaller, so that the magnetic flux of the coil 20 is unlikely to pass the magnetic paths. As a result, the printing pressure is greatly different between when a single printing wire is driven and when a plurality of printing wires are driven, which leads to non-uniformity of printing density.